Ceramides, one of sphingolipids, are lipids contained in very small amount in the whole body of living organism, but they account for at least half of the intercellular lipids in the horny cell layer that is the outermost layer of skin layers, and they play an important role in the skin's barrier mechanism. The ceramides function after they were produced and secreted in epidermal cells to constitute an intercellular lamellar structure in a horny cell layer.
However, as to skin diseases such as dry skin, irritated skin, atopic dermatitis, senile xerosis and psoriasis, there have been many reports that healthy metabolism of ceramides is disturbed and ceramides content in horny cell layer is decreased, thereby causing the decrease in the skin's moisturizing function and barrier function.
For treating such skin diseases, a method of supplementing externally natural ceramides of animal or plant origin and of microorganism origin, chemically synthesized ceramides and/or ceramide analogues has been investigated. However, due to small amount of ceramides contained in each individual of animals, plants and microorganisms, ceramides are difficult to isolate and expensive. Certain types of ceramides having a specified structure are industrially produced through chemical synthesis. However, the production involves problems with respect to the function of the ceramide produced and expensive production cost. On the other hand, ceramide analogues having functions similar to those of ceramides have been studied and reported (e.g., Patent Document 1, etc.), but their effects were not necessarily satisfactory.
Meanwhile, it has been known that certain compounds of alkyl succinic acid diamide exhibit a surfactant effect and the like (Patent Document 2).
It has not been known, however, that a specific derivative of succinic acid diamide shows a ceramide-like effect.